1. Technical Field
The invention relates generally to roller cone drill bits for drilling earth formations, and more specifically to roller cone drill bit designs.
2. Background Art
Roller cone rock bits and fixed cutter bits are commonly used in the oil and gas industry for drilling wells. FIG. 1 shows one example of a roller cone drill bit used in a conventional drilling system for drilling a well bore in an earth formation. The drilling system includes a drilling rig 10 used to turn a drill string 12 which extends downward into a borehole 14. Connected to the end of the drill string 12 is a roller cone-type drill bit 20, shown in further detail in FIG. 2.
Referring to FIG. 2, roller cone drill bits 20 typically comprise a bit body 22 having an externally threaded connection at one end 24, and a plurality of roller cones 26 (usually three as shown) attached at the other end of the bit body 22. The cones 26 are able to rotate with respect to the bit body 22. Disposed on each of the cones 26 of the bit 20 is a plurality of cutting elements 28 typically arranged in rows about the surface of each cone 26.
The cutting elements 28 on a cone 26 may include primary cutting elements, gage cutting elements, and ridge cutting elements. Primary cutting elements are the cutting elements arranged on the surface of the cone such that they contact the bottomhole surface as the bit is rotated to cut through the formation. Gage cutting elements are the cutting elements arranged on the surface of the cone to scrape the side wall of the hole to maintain a desired diameter of the hole as the formation is drilled. Ridge cutting elements are miniature cutting elements typically located between primary cutting elements to cut formation ridges that may pass between the primary cutting elements to protect the cones and minimize wear on the cones due to contact with the formation. The cutting elements 28 may be tungsten carbide inserts, superhard inserts, such as polycrystalline diamond compacts, or milled steel teeth with or without hardface coating.
Typically, roller cone bits, especially bits with milled steel teeth, include one or more cutting elements arranged about the apex of at least one cone to cut through formation near the center of the bit. The cone apex having cutting elements arranged thereon is commonly referred to as a xe2x80x9cspearpointxe2x80x9d of the bit. One example of a spearpoint on one cone of a roller cone drill bit is shown at 114a in FIG. 3A.
Some bits exist which do not include a spearpoint to cut formation near the center of the bit. These bits are commonly referred to as xe2x80x9ccoring bitsxe2x80x9d and are used for drilling a borehole with an uncut center (or core) within the hole. Coring bits differ from conventional roller cone bits in that coring bits are purposefully designed to form a core within in the borehole as the borehole is drilled. On the other hand, conventional roller cone bits are designed to drill the entire formation in the borehole, wherein formation near the center of the bit is drilled by the spearpoint of the bit, typically located at the apex of one cone.
Significant expense is involved in the design and manufacture of drill bits to produce bits which have increased drilling efficiency and longevity. For more simple bit designs, such as those for fixed cutter bits, models have been developed and used to design and analyze bit configurations which exhibit balanced forces on the individual cutting elements of the bit during drilling. Fixed cutter bits designed using these models have been shown to provide faster penetration and long life.
Roller cone bits are more complex than fixed cutter bits, in that the cutting surfaces of the bit are disposed on roller cones, wherein each roller cone independently rotates relative to the rotation of the bit body about an axis oblique to the axis of the bit body. Because the cones rotate independently of each other, the rotational speed of each cone of the bit is likely different from the rotation speed of the other cones. The rotation speed for each cone of a bit can be determined from the rotational speed of the bit and the effective radius of the xe2x80x9cdrive rowxe2x80x9d of the cone. The effective radius of the drive row is generally related to the radial extent of the cutting elements that extend axially the farthest from the axis of rotation of the cone, these cutting elements generally being located on a so-called xe2x80x9cdrive rowxe2x80x9d. Adding to the complexity of roller cone bit designs, the cutting elements disposed on the cones of the roller cone bit deform the earth formation by a combination of compressive fracturing and shearing. Additionally, most modern roller cone bit designs have cutting elements arranged on each cone so that cutting elements on adjacent cones intermesh between the adjacent cones, as shown for example in FIG. 3A and further detailed in U.S. Pat. No. 5,372,210 to Harrell. Intermeshing cutting elements on roller cone bits is desired to permit high insert protrusion to achieve competitive rates of penetration while preserving the longevity of the bit. However, intermeshing cutting elements on roller cone bits substantially constrains cutting element layout on the bit, thereby, further complicating the designing of roller cone drill bits.
Because of the complexity of roller cone bit designs, accurate models of roller cone bits have not been widely developed or used to design roller cone bits. Instead, roller cone bits have largely been developed through trial and error. For example, if cutting elements on one cone of a prior art bit are shown to wear down faster that the cutting elements on another cone of the bit, a new bit design might be developed by simply adding more cutting elements to the faster worn cone in hopes of reducing the wear of each cutting element on that cone. Trial and error methods for designing roller cone bits have led to roller cone bits which have an imbalanced distribution of force on the bit. This is especially true for roller cone bits which have cutting elements arranged to intermesh between adjacent cones and a spearpoint on one of the cones.
One example of a prior art bit considered effective in the drilling wells is shown in FIGS. 3A-3D. This drill bit comprises abit body 100 and three roller cones 110 attached thereto, such that each roller cone 110 is able to rotate with respect to the bit body 100 about an axis oblique to the bit body 100. Disposed on each of the cones 110 is a plurality of cutting elements 112 for cutting into an earth formation. The cutting elements are arranged about the surface of each cone in generally circular, concentric rows substantially perpendicular to the axis of rotation of the respective cone as illustrated in FIG. 3C. In FIG. 3A, the profiles of each row of cutting elements on each cone are shown in relation to each other to show the intermeshing of the cutting elements between adjacent cones. In this example, the cutting elements comprise milled steel teeth with hardface coating applied thereon. This type of drill bit is commonly referred to as a xe2x80x9cmilled toothxe2x80x9d bit.
As is typical for modem milled tooth roller cone bits, the teeth of the bit are arranged in three rows 114a, 114b, and 114c on the first cone 114, two rows 116a and 116b on the second cone 116, and two rows 118a and 118b on the third cone 118. As shown in FIG. 3A, the teeth of the bit are arranged on the cones such that at least one row of teeth on each cone intermeshes with a row of teeth on an adjacent cone.
As is typically for milled tooth roller cone bits, the first row of teeth 114a on the first cone 114 is located at the apex of the cone to cut formation at the center of the bit, proximal to the bit axis of rotation, as shown in FIG. 3B. This row of teeth located at the apex of the first cone is referred to as the spearpoint of the bit, as described above. To avoid contact with the spearpoint on the first cone, the apexes of the other two cones 116, 118 are truncated.
While roller cone drill bits with spearpoints are generally considered effective in drilling well bores, spearpoints have also been shown to result in large moments on the bit due to the force on the tip of the spearpoint resulting from contact with the formation during drilling. In general, the longer the spearpoint with respect to the other cones, the larger the moment arm and resulting moment. Thus it is desirable to provide a roller cone drill bit which cuts through formation at the center of the bit without the use of a spear point.
The invention comprises a roller cone drill bit for drilling an earth formation. The drill bit includes a bit body and a plurality of roller cones attached to the bit body and able to rotate with respect to the bit body. Each roller cone of the bit includes an truncated apex and a side surface. The drill bit further includes a plurality of cutting elements disposed on the side surface of each cone. The cutting elements on at least one cone are arranged such that at least one cutting element on that cone extends past an axis of rotation of the bit body as the bit is rotated. In one embodiment, the drill bit includes three cones and the cutting elements are arranged on the cones so that cutting elements on adjacent cones intermesh between the cones.